Differential bus drivers

Abstract: This invention relates to differential bus drivers for use in, for example, communication systems. The driver achieves highly symmetrical wave forms at the output stage for both high and low side drivers. In addition, the layout of components of the high and low side drivers is substantially identical which allows production of the driver as an integrated circuit with a simple layout.

Description: This invention relates to Differential Bus Drivers for use in, for electronic communications systems. In electronic communication systems such as, for example, Automotive multiplex wiring, computer interfaces, etc, there are often one or more data buses connecting subcircuits of the system. In order for a signal to be transmitted from the data buses to either internal or external subcircuits of the system, Bus drivers are generally utilised. One such type of Bus driver circuit is a Differential bus Driver. Differential bus drivers are generally incapable of producing symmetrical outputs when operating with both current sink and current source capabilities. In addition, the circuitry required to facilitate the two types of capabilities is often fairly complex in layout and design. In US-A-4 959 561, there is described an output buffer circuit using separate high and low driving circuits of complementary design and as set out in the preamble portion of Claim 1. In each driving circuit a monitoring transistor receives the same drive signal as the output transistor and a load resistor of the monitoring transistor provides feedback to the drive signal conductor through a gate also receiving an input data signal. In the case of the high driving circuit the gate is a NAND gate and in the case of the low driving circuit the gate is a NOR gate. The feedback to the drive signal conductor serves to reduce the effect on the buffer circuit of a disturbance on the power supply line to the circuit. Accordingly one object of the present invention is to provide a differential bus driver which achieves highly symmetrical waveforms. Another object of the invention is to provide a differential bus driver which is as applicable to voltage mode interface standards such as RS422 Controller Area Network (CAN), Automotive Multiplex Wiring System (Abus) as it is to current mode standards like Vehicle Area Network (VAN). According to the present invention there is provide a bus driver circuit as defined in Claim 1. Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a circuit diagram of a low side driver according to the present invention; Figure 2 is a circuit diagram of a high side driver according to the invention; and Figure 3 is a circuit diagram of a preconditioning circuit for the driver of Figure 1 or Figure 2. The circuits of Figure 1 and 2 show respectively an output driver with current sink capability (low side driver) and output driver with current source capability (high side driver). These circuits are under current mode control. The aim of the illustrated circuits is to illustrate a current source which has identical AC and DC characteristics to a matching current sink. This is achieved by using a feedback system to control how the currents are generated. The circuits allow the use of the same amplifier scheme 2 in both the source and the sink circuit. This is possible due to the fact that in both circuits the current in the output is measured with respect to the positive rail Vcc. In one circuit 10 (the current sink) the output 12 is taken from the collector 14 of an NPN 16 and in the other circuit 18 (the current source) it is taken from the Emitter 20. Diodes are included to allow the outputs to be of high impedance to the out of rail voltages. Because both circuits use a matched PMOS 30, amplifier 2 and NPN 16 the propagation delay also is well matched. The propagation delay when the circuit is turned on can be further enhanced by forcing the gate voltage to a point which is dependent on process parameters of the NPN and PMOS using SW2. Such a reference can be made as in circuit 3. The output is turned on and off by respectively opening and closing SW1. SW1 and SW2 may be implemented using MOS switches. The circuits can be adapted for use in voltage mode control in the following manner. Slow rate can be controlled by the addition of a Capacitor C and the use of a current mirror to invert the current signal in the case of the low side. As I=C(dv/dt) if the current provided into the capacitor is controlled a controlled slew rate can be achieved. The high and low side drivers will now be described in greater detail. Both high side and low side drivers use the same components. An output NPN 16 is configured in either common collector mode (Figure 2) or in common emitter mode (Figure 1). The output stage current is monitored by a current mirror configured NPN 32 connected to the output NPN. To sense the current only a small proportion of the output stage current, for example, 1/50th is fed through the current mirror. For the high side driver, this current adds to the output stage, but can be compensated for and forms only a small error. The current in the output leg of the current mirror is converted to a voltage signal by means of a precision resistor 34, made of a material which matches to the current defining element (current reference circuit) elsewhere in the circuit. A high speed transconductance amplifier 2 then measures the voltage across the sense resistor, the amplifier differential input being referenced to a precision voltage reference 36. Thus the desired current setting can be defined either by an internal voltage source, or by an external current source. An external current source can be an inherently more precise scheme since the absolute value of current defining resistors on an IC is difficult to define precisely. In both circuits, the output NPN is driven by a PMOS transistor 30. The transconductance amplifier output drives the gate of the PMOS device. In operation, the amplifier is allowed to drive the gate of the PMOS such that the NPN delivers output current up to the limit determined by the feedback loop thus formed. Thus the output stages are current limited. To set the system frequency response, a capacitor can be added to the gate of the PMOS (which is already capacitive) and the transconductance (Gm) of the transconductance amplifier is programmed (e.g. by means of a resistor). If desired the feedback to the PMOS gate may also be influenced by the voltage on the driver output. For example, capacitive feedback from the output to the gate of the PMOS can cause the output voltage swing to the slew rate controlled. The polarity of the feedback must be adjusted depending upon the polarity of the output, i.e. depending upon whether it is high side or low side. This may be achieved using an inverter 40. Also, to control the DC output voltage swing, DC voltage feedback loops (not shown) can also be formed. This can, for example, be made to limit the saturation of the NPN output stage (in the low side driver). It is desired in one application of the driver that the output stages be primarily current controlled, and thus should present a high impedance current characteristic independent of the output voltage. For the low side driver, this is achieved by relying upon the high output impedance characteristics of conventional Integrated Circuit NPN transistors which typically have an "early voltage" of near 100V. The PMOS VDS voltage in this case remains constant, and hence the Gm characteristics of this device are not varied as the output voltage moves. However, for the high side driver, this is not the case. The NPN as before has an inherently high output impedance, but as the output voltage moves, so does the PMOS VDS voltage. This causes a dynamic change to the DC and AC characteristics of the feedback loop. The application of a very high speed transconductance amplifier largely eliminates these effects except at the extreme point as the PMOS VDS collapses to near zero. The application of these drivers requires the dynamic current and voltage behaviour of the two output stages to be as symmetrical as possible. At the point of initial turn on, the low side driver output voltage is typically at say 5V (Vsupply=5V). For the high side driver, the output voltage is zero. Thus at the point of turn ON the VDS voltages for the two PMOS devices are almost identical. A preconditioning circuit (a scheme of which is shown in Figure 3 and described in more detail below) defines an "ideal" aiming voltage that the PMOS gate should be at in order to generate the required current on the output stage, and is approximately the same VDS for either high or low side drivers. At the point of turn on, this voltage is impressed upon the output of the transconductance amplifier, to provide a rapid turn on from zero output current to maximum output current. Typically, the output drivers would be loaded by a passive cable having a resistive and capacitive load; as the driver stages turn on, thus the output voltages change according to the current generated and the load impedance characteristics (Ohms law applies). If this is primarily capacitive the constant current driver output stages cause a linear increase in output voltage over time. Thus for the high side driver, the PMOS VDS changes. The feedback loop frequency response is optimised such that the loop speed is relatively fast compared to the required transition speed of the output voltage wave form. Thus the changing VDS of the high side PMOS has little impact upon the eventual current and voltage wave forms generated. Referring to the preconditioning circuit, this can be designed in many ways. One preferred embodiment of the circuit 50 is shown in Figure 3. The circuit comprises a current source 52 which generates a current that after processing is output via switch 54 to the PMOS 30 in Figures 1 and 2. The circuit 50 further includes a PMOS 56, an NPN transistor 58 and three PNP transistors 60, 62, 64 and a resistor 66. Transistors 64 has a rectifying function whilst the other components all form part of a feed back loop which controls the Vbe on transistor 64. The high side and low side drivers schemes can be generated from identical circuits, simply changing the output node connections. Various blocking diodes may be required and these would be replicated for both types of driver to ensure the same parasitics apply. Similar considerations can be applied to any other specific circuit modifications required for either high or low side driver. Since both circuits are essentially the same, identical structures and whole cells they can be implemented on an integrated circuit. The identical circuits, scheme of operation and matching layouts all help to ensure the eventual output wave forms are as symmetrical as possible. Voltage mode standards provide control over the wave shapes by limiting the maximum dv/dt and in current mode systems the outputs act like current sources and sinks and the wave forms are then controlled by the line impedance.

Claims: A bus driver circuit comprising: a low side driver and a high side driver, wherein each of said drivers has a first transistor (16) responsive to a first signal at its control electrode to provide a drive current to a terminal (12) common to the two drivers and connectible to a bus; a second transistor (32) connected to receive the first signal at its control electrode to provide in response to that signal a monitor signal current; a resistor (34) for converting the monitor signal current into a monitor signal voltage; an amplifier (2) responsive to the difference between a reference potential (36) and the monitor signal voltage to provide a control signal dependent on the difference, and a third transistor (30) connected to receive an input signal at a control electrode thereof and connected to supply the first signal to the control electrodes of the first and second transistors (16,30) so that in operation the first transistor (16) provides the drive current, and the second transistor (32) maintains the drive current at a predetermined value, characterised in that said third transistor also receives the said control signal at its said control electrode. A circuit according to claim 1, wherein the control electrodes of the first and second transistors (16,32) are connected together and reference electrodes of the first and second transistors are connected to a common conductor through respective resistors, so that the currents through the first and second transistors are related by the inverse of the relationship between the resistors. A circuit according to claim 2, wherein the first and second transistors (16,32) are NPN bipolar transistors, of which the bases are the control electrodes and the emitters are the reference electrodes. A circuit according to any one of the preceding claims in which the third transistor (30) is a field effect transistor, the input signal and the control signal being applied to the gate of the field effect transistor, and the input signal is a switched current. A circuit according to any one of the preceding claims in which each of the high side driver and the low side driver includes a capacitor (c) connected from the terminal (12) to the output of the amplifier (2), a current mirror (40) being included in the low side driver to invert the current signal through the capacitor of that driver. A circuit according to any one of the preceding claims wherein the input signal to each of the high side driver and the low side driver is provided by a preconditioning circuit (50). A circuit according to claim 6, wherein the preconditioning circuit (50) is arranged to define the voltage at the control electrode of the third transistor (30) for generating a desired output current at the terminal (12). A circuit according to claim 3, wherein the first transistor of the low side driver is configured in common emitter mode. A circuit according to claim 3 or claim 8, wherein the first transistor of the high side driver is configured in common collector mode. A circuit according to any one of the preceding claims fabricated as a single integrated circuit. A circuit according to any one of the preceding claims, wherein the propagation delays of the high side driver and the low side driver are substantially the same.

Inventors: CORSI MARCO, FATTORI FRANK, CORSI,MARCO, FATTORI, FRANK
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS LTD, TEXAS INSTRUMENTS INCORPORATED, TEXAS INSTRUMENTS LIMITED
Application year: 1994
Main classification: H03K19
IPC codes: H03K19, G06F13, H04L25